# C.8.19.6.13.1 Isocenter Reference System Attribute Description

The Isocenter Reference System Attributes describe the 3D geometry of the X-Ray equipment composed by the X-Ray positioner and the X-Ray table.

These attributes define three coordinate systems in the 3D space:

• Isocenter coordinate system

• Positioner coordinate system

• Table coordinate system

The Isocenter Reference System attributes describe the relationship between the 3D coordinates of a point in the table coordinate system and the 3D coordinates of such point in the positioner coordinate system (both systems moving in the equipment), by using the Isocenter coordinate system that is fixed in the equipment.

Note: PS 3.17 Annex Enhanced XA/XRF Encoding Examplesdescribes the transformations necessary to transpose between coordinate systems.

C.8.19.6.13.1.1 Isocenter Coordinate System

The Isocenter coordinate system (O,X,Y,Z) of the equipment is defined as follows:

• Origin O is on the System Isocenter

• +Y DOWNWARD (gravity)

• +X, +Z directions in the horizontal plane (gravity plane). Directions arbitrarily defined by the manufacturer

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Figure C.8.19.6-4Isocenter Coordinate System

C.8.19.6.13.1.2 Positioner Coordinate System

The positioner coordinate system (O p , X p , Y p , Z p ) is defined as follows:

• Origin O p , is the origin of the Isocenter coordinate system O

• X p axis is parallel to the horizontal scan-lines of the detector (rows). Positive direction from left to right of the detector plane looking towards the source.

• Z p axis is parallel to the vertical scan-lines of the detector (columns). Positive direction from bottom to top of the detector plane looking towards the source.

• Y p is the axis from the isocenter to the X-Ray source. Positive direction from the Isocenter to the X-Ray Source. This axis is so-called the X-Ray center beam.

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Figure C.8.19.6-5 Positioner Coordinate System

Note: The quantities SID and ISO are specified by the attributes Distance Source to Detector (0018,1110) and Distance Source to Isocenter (0018,9402) respectively.

The Positioner coordinate system (O p , X p , Y p , Z p ) is characterized, with respect to the Isocenter coordinate system (O, X, Y, Z), by two angles describing the X-Ray center beam, and a third angle describing the rotation of the X-Ray detector plane. These angles are relative to the Isocenter reference system, and independent from the patient position on the equipment.

Positioner Isocenter Primary Angle (0018,9463) (so-called Ap 1 in Figure C.8.19.6-6) is defined in the plane XY, as the angle between the plane YZ and the plane Y p Z. The axis of rotation of this angle is the Z axis. Angle from -Y to +X is positive. The valid range of this angle is -180 to +180 degrees.

Positioner Isocenter Secondary Angle (0018,9464) (so-called Ap 2 in Figure C.8.19.6-6) is defined in the plane Y p Z, as the angle of the X-Ray Center Beam (i.e. Y p ) relative to the XY plane. The axis of rotation of this angle is perpendicular to the plane Y p Z. Angle from the plane XY to +Z is positive. The valid range of this angle is -180 to +180 degrees.

Positioner Isocenter Detector Rotation Angle (0018,9465) (so-called Ap 3 in Figure C.8.19.6-6 and in Figure C.8.19.6-7) is defined in the detector plane, as the angle of the vertical scan-lines of the detector (i.e. Z p ) relative to the intersection between the detector plane and the plane Y p Z. The sign of this angle is positive clockwise when facing on to the detector plane (see Figure C.8.19.6-7). The valid range of this angle is -180 to +180 degrees.

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Figure C.8.19.6-6Positioner Isocenter Angles

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Figure C.8.19.6-7Positioner Isocenter Detector Rotation Angle when Ap 1 =0 and Ap 2 =0

C.8.19.6.13.1.3 Table Coordinate System

The table coordinate system (O t , X t , Y t , Z t ) is defined as follows:

• Origin O t , so-called Table Reference Point, is on the Table Top plane

• +X t direction to the TABLE LEFT

• +Z t direction to the TABLE HEAD

• +Y t direction to the TABLE DOWN

The table coordinate system (O t , X t , Y t , Z t ) is characterized, with respect to the Isocenter coordinate system (O, X, Y, Z), by a 3D translation and 3 angles describing the tilting and rotation:

Table X Position to Isocenter (0018,9466) (so-called T X in Figure C.8.19.6-8) is defined as the translation of the Table Reference Point O t with respect to the Isocenter coordinate system in the X direction. Table motion toward +X is positive.

Table Y Position to Isocenter (0018,9467) (so-called T Y in Figure C.8.19.6-8) is defined as the translation of the Table Reference Point O t with respect to the Isocenter coordinate system in the Y direction. Table motion toward +Y is positive.

Table Z Position to Isocenter (0018,9468) (so-called T Z in Figure C.8.19.6-8) is defined as the translation of the Table Reference Point O t with respect to the Isocenter coordinate system in the Z direction. Table motion toward +Z is positive.

Note: A translation of ( T X, T Y, T Z ) = (0, 0, 0) means that the Table Reference Point O t is at the System Isocenter.

Table Horizontal Rotation Angle (so-called At 1 in Figure C.8.19.6-9) is defined in the horizontal plane XZ, as the angle of the projection of the +Zt axis in the XZ plane relative to the +Z axis. The axis of rotation of this angle is the vertical axis crossing the Table Reference Point Ot. Zero value is defined when the projection of +Zt in the XZ plane is equal to +Z. Angle from +Z to +X is positive. The valid range of this angle is -180 to +180 degrees.

Table Head Tilt Angle (so-called At 2 in Figure C.8.19.6-9) is defined in the vertical plane containing Z t (i.e. YZ t ), as the angle of the +Z t axis relative to the horizontal plane XZ. The axis of rotation of this angle is defined as the intersection between the horizontal plane XZ and the plane X t Y t . Zero value is defined when +Z t is contained in the horizontal plane XZ. Angle from horizontal (plane XZ) to -Y direction (upwards) is positive, indicating that the head of the table is above the horizontal plane. The valid range of this angle is -45 to +45 degrees.

Table Cradle Tilt Angle (so-called At 3 in Figure C.8.19.6-9) is defined in the X t Y t plane, as the angle of the +X t axis relative to the intersection between the X t Y t plane and the horizontal plane XZ. The axis of rotation of this angle is the axis Z t . Zero value is defined when +X t is contained in the horizontal plane XZ. Angle from horizontal (plane XZ) to -Y direction (upwards) is positive, indicating that the left of the table is above the horizontal plane. The valid range of this angle is -45 to +45 degrees.

Note: The angles At 1 , At 2 and At 3 are independent from any specific mechanical design of the table rotation axis defined by a manufacturer. In particular, they donâ€™t require the three rotation axis to cross on a single point. If a mechanical rotation axis does not cross the Table Reference Point O t , a mechanical rotation around this axis will generate a change in one or more table angles as well as a translation of the Table Reference Point.

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Figure C.8.19.6-8 Table Translation with respect to the Isocenter Reference System

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Figure C.8.19.6-9 Table Angulations with respect to the Isocenter Reference System